Yes they're interchangeable, but ancestors is more recent and more widely used and recognised.
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spiceyokookoJan 15 '13 at 16:37

Welcome to EL&U. Your question should include your research on the matter -- in this case, for example, dictionaries you consulted and why the result was inadequate. Thanks.
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MετάEdJan 15 '13 at 16:48

In programming ascendants is sometimes used in technical lingo to refer to hierarchically organized "things" which do not (necessarily) represent people. So in programming ascendants is used as a more generic concept than ancestors. Ex.: categories (and other taxonomies) may have a parent, children, ascendants, descendants. Other than that I perceive them as full synonyms -- and I am aware that ascendants is far less commonplace.
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Mihai StancuJul 25 at 23:02

I think what you mean is antecedent vs. ancestor. I am not sure if ascendent connotes the meaning you have in mind, but the two others I suggested would be interchangeable in some contexts. However, in your specific case ancestor seems to be a better fit.

Ascendant/ascendent (both spellings are found) can mean what the querent asks about here too. It's just so rare as to be ill-advised.
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Jon HannaJan 15 '13 at 16:18

From OED: Ascendent/ascendant: One who precedes in genealogical succession; an ancestor; a relative in the ascending line, whether lineal, as father, mother, or collateral, as uncle, great-uncle.
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spiceyokookoJan 15 '13 at 16:35